A digital memory cell device of this type can magnetically store. An individual memory cell device is generally part of a memory device, often also called an MRAM (magnetic random access memory). A memory of this type can be used to carry out read and/or write operations. Each individual memory cell device comprises a soft-magnetic read and/or write layer system, which is separated by an intermediate layer from a hard-magnetic reference layer system formed as an AAF system. The magnetization of the reference layer of the reference layer system is stable and does not change in an applied field, while the magnetization of the soft-magnetic read and/or write layer system can be switched by an applied field. The two magnetic layer systems can be magnetized parallel or antiparallel with respect to one another. The two aforementioned states in each case represent a bit of information, i.e., the logic zero (“0”) state or one (“1”) state. If the relative orientation of the magnetization of the two layers changes from parallel to antiparallel, or vice versa, then the magnetoresistance across this layer structure changes by a few percent. This change in resistance can be used for the read-out of digital information stored in the memory cell. The change in the cell resistance can be identified by a voltage change. By way of example, the cell may be occupied by a logic zero (“0”) in the event of a voltage increase and the cell may be occupied by a logic one (“1”) in the event of a voltage decrease. Particularly large resistance changes in the region of a few percent have been observed when the magnetization orientation changes from parallel to antiparallel and vice versa in cell structures of the GMR type (giant magnetoresistance) or the TMR type (tunnel magnetoresistance).
An important advantage of such magnetic memory cells is that the information is stored in a persistent manner, and is stored without maintenance of any basic supply even with the device switched off and is immediately available again after the device is switched on, unlike in known conventional semiconductor memories.
A central component is the reference layer system formed as an AAF system (AAF=artificial antiferromagnetic). Such an AAF system is advantageous on account of its high magnetic rigidity and the relatively low coupling to the read and/or write layer system through the so-called orange peel effect and/or through macroscopic magnetostatic coupling fields. An AAF system generally comprises a first magnetic layer or a magnetic layer system, an antiferromagnetic coupling layer and a second magnetic layer or a magnetic layer system which is coupled by its magnetization, via the antiferromagnetic coupling layer, oppositely to the magnetization of the lower magnetic layer. Such an AAF system can be formed, e.g., from two magnetic Co layers and an antiferromagnetic coupling layer made of Cu.
In order to improve the rigidity of the AAF system, that is to say its resistance to external outer fields, it is customary to arrange an antiferromagnetic layer at the magnetic layer of the AAF system which is remote from the read and/or write layer system. By means of this antiferromagnetic layer, the directly adjacent magnetic layer is additionally pinned in its magnetization, so that overall the AAF system becomes harder (exchange pinning or exchange biasing).
The magnetic rigidity of the AAF system corresponds to the amplitude of the applied external fields, which is required for rotating the magnetizations of the two ferromagnetic layers in the same direction, i.e., for parallel setting. This limits the magnetic window for read and write applications of such a memory cell device.